Saimiri oerstedii [Central American Squirrel Monkey] Although squirrel monkeys are abundant and widespread in South America, The IUCN rates the Central American squirrel monkey as vulnerable. It occurs in a region, the Pacific wet lowlands of Costa Rica and Panama, widely separated from the ranges of other squirrel monket ys. Some experts believe S. oerstedii is a hybrid population introduced by pre-Colombian Amerind traders derived from distinct types of squirrel monkeys introduced from various localities in South America. On the basis of nuclear and mitochondrial DNA sequence data, Cropp and Boinski (2000) felt they had rejected this hypothesis. However, Chiou et al. (2011) found that a sample S. oerstedii mtDNA haplotypes were more than 99% similar to some Saimiri sciureus haplotypes, which was even more similar than were some of the other S. sciureus haplotypes evaluated in their study. This finding is consistent with, though it does not confirm, the hypothesis that is derived from hybridization involving S. sciureus females.

Saimiri sciureus [Common Squirrel Monkey]
See also: Saimiri boliviensis and "Springhare + Common Squirrel Monkey."
×Saimiri ustus [Bare-eared Squirrel Monkey] ENHR(Brazil). A male specimen in the Emilio Gouldi collection (MG13210) taken on the right bank of the Rio Tefé at the mouth of Rio Bauana (Ponta da Castanha: see Ayres 1985, Map 1) is thought to be this hybrid. It has the fulvous hands and burnt-orange dorsum of ustus, but the gray auricular patch of sciureus. The Museu de Zoologia has additional probable hybrids (MZ19013, MZ19014, MZ19015) collected on the Rio Tefé’s east bank (from Vila Vale). Other intermediate specimens were collected between the Rio Madeira and Rio Tapajós. Costello et al. 1993 (pp. 196-198).

Saimiri ustus [Bare-eared Squirrel Monkey]
See also: Saimiri boliviensis; S. sciureus.
× Saimiri vanzolinii [Black Squirrel Monkey] A female specimen in the Emilio Gouldi collection (MG13209) taken at Lago Boia (w bank of the Rio Tefé, Brazil) is thought to be either this hybrid or Saimiri boliviensis × S. ustus. The former however, was thought to be more likely due to the proximity in that region of the putative parents. Costello et al. 1993 (pp. 196-197).

Saimiri vanzolinii [Black Squirrel Monkey] See: Saimiri boliviensis; S. ustus. This monkey lives in central Amazonia in the várzea forest at the confluence of the Japura and Solimoes rivers (see map above). No one seems to have suggested it, but the possibility should be investigated that vanzolinii may be of hybrid origin. It has a tiny range in comparison with the other three commonly recognized South American squirrel monkeys (Saimiri boliviensis, Saimiri sciureus and Saimiri ustus) and it occurs at the common intersection of their three ranges (see map above). These facts are consistent with the idea that it may simply be the product of a three-way hybrid zone, a very common phenomenon with other types of animals. S. sciureus is already known to hybridize in a state of nature with both S. boliviensis and S. ustus. And, the latter two, which are very similar animals, will likely also turn out to hybridize once their remote zone of contact is investigated. As Costello et al. (1993, p. 206) point out, no pre-mating behaviors are known that might reproductively isolate “any one Saimiri group from any other. All squirrel monkeys share the same feeding strategy, diet, and breeding systems.”